Abstract Errors in chromosome segregation are the major source of aneuploidy and a driving force in the development of tumors. The spindle assembly checkpoint (SAC) is one of the mechanisms that prevents aneuploidy and is an important regulator of genomic stability. The SAC is orchestrated by the kinetochore and a major unresolved problem, addressed in this proposal, is to determine how the kinetochore measures the lack of tension and lack of occupancy and how the kinetochore transfers that information to the cell cycle machinery. We will complete high resolution mapping of the tension branch of the SAC within the kinetochore using loss-of- function mutants. We have developed a phospho-specific antibody to Mad3 that is the first biochemical marker for the tension branch of the SAC and we propose to use it to dissect this important component of SAC regulation. We propose a new model for the role of the kinetochore in the tension checkpoint. We will determine if subsets of Mad3-containing proteins are phosphorylated, where Mad3 is phosphorylated in the cell and which kinetochore proteins are needed to transmit the tension signal. We have isolated novel mutants that are specific to the occupancy branch of the SAC. We will dissect the role for the SAC in the DNA damage pathway which requires Mec1 and Tel1 phosphorylation of SAC proteins and represents a novel pathway of mitotic regulation. Together, the experiments in this proposal will provide important insights into how SAC signaling is initiated and integrated into the cell cycle.